linux-kernel - Re: ABI coupling to hypervisors via CONFIG

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Date:	Fri, 9 Mar 2007 22:04:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Zachary Amsden <zach@...are.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>, akpm@...ux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Chris Wright <chrisw@...s-sol.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: ABI coupling to hypervisors via CONFIG_PARAVIRT

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> So please
> 
>  - point out things that are badly done. [...]

the thing badly done is fundamental and it trumps any other small 
technological detail complaint i have, because it affects the 
development and maintainance model: to promise backwards compatibility 
to 4-5 different hypervisors, using separate ABIs for each. /One/ such 
ABI would be complex enough to maintain IMO.

( if there is no backwards compatibility promise then i have zero
  complaints: then paravirt_ops + the hypercall just becomes another API
  internal to Linux that we can improve at will. But that is not
  realistic: if we provide CONFIG_VMI today, people will expect to have
  CONFIG_VMI in the future too. )

as i said in the very, very first email about this topic: /one/ new ABI 
towards hypervisors should be introduced step by step, and via concensus 
across hypervisors. We should treat the hypercall ABI very similar to 
the system call ABI: the system call ABI is largely based on a concensus 
between applications.

[ i think apic_write() granularity is bad too - but that is a small
  technical issue, dwarved by the ABI issues that impact the development
  model IMO. ]

	Ingo
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